This invention pertains to a tensioning device or slip clutch. In one of its more specific aspects, the invention relates to a tensioning device utilizing a permanent magnet and a magnetic coupling provided by magnetic particles to obtain an adjustable torque slip clutch.
Tensioners or slip clutches have been used for a number of years in industrial uses. One function of these tensioners or clutches is to provide a certain level of torque or resistance to turning in machinery where material is being wound or unwound on a mandril. In the past, friction clutches have been frequently used to provide the desired resistance to turning. However, friction clutches have a disadvantage because the friction material slips or abrades and gradually wears out. Thus, the life of the friction clutch can be fairly short and is dependent upon the level of abrasion produced as the friction surfaces slip over one another. Another factor that limits the usefulness of this type of clutch arrangement is that as the friction surfaces wear the surfaces often do not provide the same amount of torque or resistance to turning. Therefore, the torque or resistance to turning can vary in the friction clutch due to the amount of wear received by the friction surfaces of the clutch. Another difficulty is that a powdery material can be produced as the friction surfaces wear. The powder is usually contained within the friction clutch mechanism and frequently causes difficulty with the operation of the friction clutch.
An electrically magnetized coil can be used to provide the desired resistance to turning. However, it is frequently difficult to provide the electrically wiring and power required to energize the magnetizable coil. The difficulties associated with an electrially magnetized coil are further compounded in areas where electrical power is not readily available or where power outages occur during the operation of the coil. Clearly such difficulties render the electrically magnetized coil virtually useless as a tensioning device or slip clutch.
A permanent magnet and the magnetic flux from the magnet can be used to link two coupling elements to provide the desired resistance to turning. The difficulty with using a permanent magnet is that it is very difficult to adjust the magnetic flux of the magnet to vary the torque or resistance to turning produced by the magnet. In the past, flux gates, as shown in U.S. Pat. No. 3,822,290, have been used to vary the magnetic flux produced by the permanent magnet. However, flux gates have the disadvantage in that they reduce the maximum torque capability for a given size magnet. Thus, to produce a tensioning device having the desired torque range it would be necessary to use a much larger permanent magnet if a flux gate is used as the means for varying the output torque of the magnet.
An adjustable magnetic shunt can also be positioned at the center of the magnet as a means for adjusting the flux of the magnet. However, to affect a large change in the working flux of the magnet, it is necessary to make the area of the shunt a large fraction of the area of the magnet. The large size of the magnetic shunt adversely effects the relationship between the torque output and the size of the tensioning device. In other words the use of a magnetic shunt has the disadvantage of inherently increasing the size of the tensioner to provide the desired range of torque or resistance to turning.